1. Field of the Invention
The present invention relates to a method and apparatus for polishing a wafer such as a semiconductor wafer, and more particularly to a technique for polishing only an outer circumferential area as a limited area of the wafer.
2. Description of the Related Art
In a semiconductor device fabrication process including the steps of forming many devices on the front side of a wafer formed of a semiconductor such as silicon and dividing the wafer to obtain the individual devices, the back side of the wafer is ground to reduce the thickness of the wafer. Such a reduction in thickness of the wafer is made in response to a desired reduction in thickness of a device package. For example, the thickness of the wafer is reduced from about 700 μm to about 200 μm. However, in response to a recent remarkable reduction in thickness of a device package, there is a case that the thickness of the wafer is reduced to 50 μm or 30 μm.
In general, the back side of a wafer is ground by a method including the steps of rotating a grinding tool such as an abrasive member and bringing the rotating grinding tool into pressure contact with the back side of the wafer. However, the ground surface of the wafer on the back side thereof after grinding has a strain layer having a thickness of about 1 μm due to minute flaws by the grinding. The strain layer causes a reduction in die strength of the wafer, and it is therefore necessary to remove the strain layer, thereby maintaining the strength of the wafer. Known as means for removing the strain layer is a technique of polishing the ground surface of the wafer by using a disk-shaped polishing member containing abrasive grains (see Japanese Patent Laid-Open No. 2003-53662, for example).
The above publication describes that the whole of the back side of the wafer is polished. As described in this publication, the whole of the back side of the wafer can be polished by bringing the polishing member having a size capable of covering the ground surface of the wafer into pressure contact with the wafer being rotated as relatively moving the polishing member parallel to the wafer. Accordingly, no accurate control is required for this parallel movement of the polishing member.
In the field of wafer polishing, there is a case of polishing only an annular outer circumferential polishing area set on the back side of a wafer with a predetermined width from the outer circumferential edge of the wafer. Such polishing can be performed by relatively moving the polishing member from the outer circumferential edge of the wafer being rotated toward the center of the wafer by the predetermined width with high accuracy. However, such accurate movement of the polishing member requires accurate measurement of correlative positions of the polishing member and the wafer and control of the movement of the polishing member according to measured values, causing hard and complicated work. Further, unless the polishing member is formed into a completely round shape about a rotation axis, the outer circumferential polishing area having the predetermined width cannot be polished in spite of accurate parallel movement of the polishing member. The formation of such a completely round shape from the polishing member is also complicated and a simple method is therefore desired.